The healthy immune system depends upon a balance of proliferation and death of white blood cells, including T cells, to contain and eliminate infectious pathogens effectively without doing unintended damage to self tissues. One way that activated T cells are instructed to die occurs upon repeated engagement of the T cell receptor (TCR); a process known as restimulation-induced cell death (RICD). This self-regulatory death program protects against excessive expansion of T cells as an immune response unfolds. The long-term objective of this project is to define specific biochemical signals that convert the TCR signal from proliferation and survival to death, which to date remain poorly understood. We previously discovered a novel RICD defect in T cells from patients with X-linked lymphoproliferative disease (XLP), which lack expression of SLAM-associated protein (SAP). SAP, in conjunction with the SLAM family receptor protein NTB-A, is required for proper RICD in normal T cells. Using a variety of genetic and biochemical approaches in human cells, this project aims to further elucidate and manipulate the molecular mechanism by which SAP and NTB-A direct TCR signaling for death in activated T cells. This basic research will illuminate a previously unrecognized network of biochemical signals connecting SAP, NTB-A, and key kinases (e.g. LCK, DGKa), phosphatases (SHP-1), and transcription factors (FOXP3) that ultimately govern RICD sensitivity in conventional and regulatory T cells. Moreover, our proposed survey of numerous human donors will determine if and how SAP and NTB-A are meaningful biomarkers of RICD sensitivity in normal individuals, which could provide an innovative new diagnostic tool for predicting the magnitude of T cell responses to infection or vaccination. Elucidating these critical signaling events will improve our basic understanding of abnormal T cell signaling and cell death in patients with lymphoproliferative and autoimmune disorders beyond XLP. Targeting these molecular interactions should offer a new therapeutic approach to control T cell responses by manipulating RICD sensitivity. Such a strategy could be applied to numerous clinical contexts in which culling excess T cells (e.g. autoimmunity, lymphoproliferative disease) or boosting T cell responses (e.g. infection, cancer) could help ameliorate disease.